The objective of this project is to assess one aspect of host immunity towards a growing tumor. We will concentrate on the infiltration of host K cells into three different kinds of murine neoplasms. K cells belong to a heterogeneous population of normal lymphoid cells which are capable of lysing antibody-coated target cells in the absence of complement. Using cell fractionation techniques, we will identify the characteristics of K cells active against various murine tumor cells from several ascites tumors, fibrosarcomas and mammary tumors. The cytotoxic and cytostatic activities of normal K cells versus antibody-coated tumor and erythrocyte target cells will be analyzed in vitro using Cr51-release and colony-inhibition assays. We have previously shown that certain K cell activities exist in ascites tumors and fibrosarcomas. K cells will be isolated directly from the tumors mentioned using cell fractionation techniques, especially velocity sedimentation at unit gravity. The anti-tumor activities of tumor-derived K cell-enriched fractions will then be studied in vitro and in vivo with purified tumor cells coated with alloantisera, syngeneic immune sera, or tumor-bearer sera. The role of immune complexes in the putative inhibition of K cell-tumor cell interactions will also be studied. It is hoped that this information will provide enough insight into antibody-dependent, cell-mediated tumor destruction in vivo to begin attempts to augment these reactions in tumor-bearing hosts.